Problem: $ { \left[\begin{array}{rr}{1} & {-1} \\ {3} & {3} \\ {1} & {1}\end{array}\right]+\left[\begin{array}{rr}{-1} & {0} \\ {-2} & {0} \\ {1} & {-1}\end{array}\right] = {?} } $
Answer: To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{rr}{1}+{-1} & {-1}+{0} \\ {3}+{-2} & {3}+{0} \\ {1}+{1} & {1}+{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{0} & {-1} \\ {1} & {3} \\ {2} & {0}\end{array}\right]}$